Memory or data storage devices are used, for example, to store program instructions that may be executed by processors. Data storage devices have also been used to store other types of data, including, for example, audio, image, and/or text information (e.g., text associated with a Microsoft Word document). Recently, systems with data storage devices capable of storing substantial data content (e.g., songs, music videos, etc.) have become widely available in portable devices (e.g., Apple's iPod and iPhone devices).
Such portable devices include data storage devices (DSDs) that have small form factors and are capable of operating from portable power sources, such as batteries. Some DSDs in portable devices may provide non-volatile memory that is capable of retaining data when disconnected from the power source. Portable devices have used various non-volatile data storage devices, such as hard disk drives, EEPROM (electrically erasable programmable read only memory), and flash memory.
Flash memory has become a widely used type of DSD. Flash memory may provide, for example, a non-volatile memory in portable electronic devices and consumer applications. Two types of flash memory are NOR flash and NAND flash. NOR flash typically provides the capacity to execute code in place, and is randomly accessible (i.e., like a RAM). NAND flash can typically erase data more quickly, access data in bursts (e.g., 512 byte chunks), and may provide more lifetime erase cycles than comparable NOR flash. NAND flash may generally provide non-volatile storage at a low cost per bit as a high-density file storage medium for consumer devices, such as, for example, digital cameras and MP3 players.
Typical flash memory stores a unit of information by storing an electrical charge in each memory cell at a voltage representative of a binary value. Single level cells store one bit of information based on the cell being charged to a “high” voltage, or being discharged to a “low” voltage.